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Figure 3.…read more

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Organelle Structure and function
Nucleus enclosed in double membrane with pores
contains chromosomes with genes made of DNA to control protein
Ribosomes made of RNA and protein
free in cytoplasm or attached to RER
site of protein synthesis
Rough endoplasmic interconnected sacs with ribosomes attached
reticulum transport proteins to other parts of cell
Smooth endoplasmic synthesis of lipids and steroids
Mitochondria double membrane ­ inner folded into cristae
site of later stages of aerobic respiration
Centrioles one pair found in animal cells
made…read more

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Proteins synthesised on the ribosomes of
the RER are moved to other parts of the
cell through the cavities of the
endoplasmic reticulum.
The Golgi apparatus is a stack of
membrane-bound sacs formed from
fused vesicles from the ER.
Proteins are modified here and packaged
in vesicles. Some eg enzymes and
hormones are released from the cell.
Figure 3.9
The cells described above, with membrane-bound organelles are eukaryotic.…read more

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Mitosis is a type of cell division, which retains the full or diploid number (2n) of chromosomes.
In humans, a cell with 46 chromosomes divides to form 2 identical daughter cells, each with 46 chromosomes.
Before nuclear division, a copy of each chromosome is made by semi-conservative replication of the DNA. Each
double helix is called a chromatid.…read more

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Cell division is a continuous process, but 4 stages of mitosis (nuclear division) can be described:
Prophase chromosomes condense (get shorter and thicker)
microtubules are organised into a spindle by the centrioles
nuclear membrane breaks down
Metaphase the centromeres of the chromosomes attach to the spindle at the equator
Anaphase centromeres split
spindle fibres pull chromatids to opposite poles
spindle breaks down
Telophase chromosomes unravel
two nuclear envelopes form
Make sure you are familiar with the details of…read more

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Gametes are produced in the ovaries and testes of animals by meiosis which:
produces haploid cells (contain half the number of chromosomes found in a body cell: one of each
homologous pair)
creates genetic variation among offspring
During meiosis, pairs of homologous chromosomes line up at the equator.
As either of the pair can end up at either pole (random assortment), genetically variable gametes are
2.3.…read more

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The specialised function of a cell depends upon the proteins it synthesises ie which genes are expressed.
Transcription of a gene is initiated by RNA polymerase and transcription factors binding to a promoter region
(section of DNA adjacent to gene).
RNA polymerase + transcription factors = transcription initiation complex
Some transcription factors are
always present in all cells.…read more

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Others are influenced by both genotype and environment and show continuous variation eg human height,
skin and hair colour, cancer.
Figure 3.38
A ­ Human blood
groups show
B ­ Human height
shows continuous
Human average height has increased in the past 150 years for various reasons.
a person may have genes for being tall, but not achieve their potential height because
of malnutrition.…read more

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Cancer occurs when the rate of cell multiplication is faster than the rate of cell death.
This causes the growth of a tumour.
Cancer is caused by environmental damage to DNA from
physical factors such as UV light and asbestos
chemical carcinogens such as those in the tar in cigarette smoke
viruses may trigger cancer by altering the DNA
Chemicals called radicals are produced by the cell metabolism and can damage DNA.
Fresh fruit and vegetables contain antioxidants to destroy radicals.…read more


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